A wired commodity vending system of this type has been known and referred to as a so-called hotel vendor system. In a conventional hotel vendor system, the control units of the master and slave stations are both constructed by logic integrated circuits having a hard wired configuration. Accordingly, the control unit has poor flexibility and is usually monofunctional. Further, the master station provided, for example, at the front desk in the hotel is wired with the slave stations in the respective guest rooms. As a result, there is a high possibility that various types of noise current and voltage are induced into the data transmission line from the power distributing lines running in parallel therewith which disturbs the transmission of the commodity sales data.
Many types of transmission systems, which accurately transmit data by preventing the inclusion of noise voltage and current, have been developed and used. General modulation systems or multi-communication systems of this type have a disadvantage in that the configuration of the necessary circuit construction is complicated and expensive for simple data transmission requirements such as employed in a wired commodity vending system, for example, a hotel vendor system.
Also, in the hotel vendor system in which the transmitted data can be sent in various ways and the cost allowed for the data transmission system is limited, it has been desired to add additional system functions with a minimum of functional construction. In the conventional hard wired system, it has been difficult to improve the freedom of adding functions while keeping the system construction inexpensive.
In the conventional hotel vendor system, each time that commodity sales data is collected by the master station, each slave station transmits its sales data only once. In this case, the data transmission rate is usually synchronized with the zero-crossing point of a commercial power source AC voltage and is limited to 100 or 120 pulses per second. By taking advantage of the power source synchronizing system, the reliability of the data transmission is improved and liquidation (processing operations based on the collected commodity sales data) occurs on the basis of single commodity sales data transmission. In a three-phase power source synchronizing system where a data pulse is transmitted at every zero cross point of the AC voltage, poor synchronization occurs due to the phase differences of each phase voltage, or a power interruption makes it impossible to transmit data. Accordingly, a three-phase AC power synchronizing system has various drawbacks.